Semiconductor devices including multilayer source/drain stressors and methods of manufacturing the same

ABSTRACT

A semiconductor device including source drain stressors and methods of manufacturing the same are provided. The methods may include forming a recess region in the substrate at a side of a gate pattern, and an inner surface of the recess region may include a first surface of a (100) crystal plane and a second surface of one of {111} crystal planes. The method may further include performing a first selective epitaxial growth (SEG) process to form a base epitaxial pattern on the inner surface of the recess region at a process pressure in a range of about 50 Torr to about 300 Torr. The method may also include performing a second selective epitaxial growth (SEG) process to form a bulk epitaxial pattern on the base epitaxial pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 to Korean Patent Application No. 10-2012-0105825, filed onSep. 24, 2012, in the Korean Intellectual Property Office, thedisclosure of which is hereby incorporated by reference in its entirety.

FIELD

The present disclosure generally relates to the field of electronics,and more particularly semiconductor devices.

BACKGROUND

Semiconductor devices are widely used in various industries such aselectronic devices, cars and/or vehicles because of their small size,lightness, and low manufacture costs. An electric field transistor(hereinafter, referred to as a transistor) may be one of importantcomponents constituting semiconductor devices. Generally, a transistormay include a source, a drain, and a gate electrode. The source and thedrain may be spaced apart from each other in a semiconductor substrate,and the gate electrode may be disposed over a channel region between thedrain and the source. The source and the drain may be formed byimplanting dopant ions into the semiconductor substrate. The gateelectrode may be electrically insulated from the channel region by agate oxide layer therebetween.

The transistors may be widely used as a switching component and/orcomponents constituting a logic circuit in the semiconductor device.Recently, high speed transistors have been increasingly demanded. On thecontrary, sizes of the transistors have been more reduced with highintegration of semiconductor devices. Thus, a turn-on current of atransistor may, be reduced and performance of the transistor maydeteriorate, such that reliability of a semiconductor device maydeteriorate. Additionally, an operating speed of the semiconductordevice may be reduced. Therefore, various researches have been conductedfor increasing the turn-on current of the transistor.

Source drain stressors may be used to increase the carrier mobility inthe channel region of the MOS transistor. Tensile stressors may be usedfor an NMOS transistor and compressive stressors may be used for a PMOStransistor. Stressor materials may be epitaxial layers.

SUMMARY

A semiconductor device may include a substrate including a firstsemiconductor element and a gate pattern on the substrate. The devicemay further include a base epitaxial pattern on an inner surface of arecess region in the substrate at a side of the gate pattern. The innersurface of the recess region may include a first surface of a (100)crystal plane and a second surface of one of {111} crystal planes. Thebase epitaxial pattern may include a second semiconductor elementdifferent from the first semiconductor element. The device may alsoinclude a bulk epitaxial pattern on the base epitaxial pattern and thebulk epitaxial pattern may include the second semiconductor element. Thebase epitaxial pattern may have a first thickness on the first surfaceand a second thickness on the second surface, and a ratio of the secondthickness to the first thickness of the base epitaxial pattern may be ina range of about ¾ to about 1.

In various embodiments, a second semiconductor element concentration inthe base epitaxial pattern may be less than a second semiconductorelement concentration in the bulk epitaxial pattern.

According to various embodiments, the substrate may include firstdopants of a first conductivity type and the bulk epitaxial pattern mayinclude second dopants of a second conductivity type different from thefirst conductivity type. A second dopant concentration in the baseepitaxial pattern may be less than a second dopant concentration in thebulk epitaxial pattern.

In various embodiments, the base epitaxial pattern may be free of thesecond dopants

According to various embodiments, the base epitaxial pattern may beformed at a process pressure in a range of about 50 Torr to about 300Torr.

In various embodiments, the recess region may include an undercut regiontapered toward a region under the gate pattern.

According to various embodiments, the semiconductor device mayadditionally include a buffer epitaxial pattern between the baseepitaxial pattern and the bulk epitaxial pattern. The buffer epitaxialpattern may include the second semiconductor element, and a secondsemiconductor element concentration in the buffer epitaxial pattern maybe less than a second semiconductor element concentration in the bulkepitaxial pattern and may be greater than a second semiconductor elementconcentration in the base epitaxial pattern.

According to various embodiments, the buffer epitaxial pattern may havea third thickness on the first surface and a fourth thickness on thesecond surface and a ratio of the fourth thickness to the thirdthickness of the buffer epitaxial pattern may be less than the ratio ofthe second thickness to the first thickness of the base epitaxialpattern.

In various embodiments, the substrate may include first dopants of afirst conductivity type, the bulk epitaxial pattern and the bufferepitaxial pattern may include second dopants of a second conductivitytype different from the first conductivity type. A second dopantconcentration in the buffer epitaxial pattern may be less than a seconddopant concentration in the bulk epitaxial pattern, and the baseepitaxial pattern may be free of the second dopants or a second dopantconcentration in the base epitaxial pattern may be less than the seconddopant concentration in the buffer epitaxial pattern.

A method of manufacturing a semiconductor device may include forming agate pattern on a substrate including a first semiconductor element andforming a recess region in the substrate at a side of the gate pattern.An inner surface of the recess region may include a first surface of a(100) crystal plane and a second surface of one of {111} crystal planes.The method may further include performing a first selective epitaxialgrowth (SEG) process to form a base epitaxial pattern on the innersurface of the recess region at a process pressure in a range of about50 Torr to about 300 Torr. The base epitaxial pattern may include asecond semiconductor element different from the first semiconductorelement. The method may also include performing a second selectiveepitaxial growth (SEG) process to form a bulk epitaxial patternincluding the second semiconductor element on the base epitaxialpattern.

In various embodiments, a second semiconductor element concentration inthe base epitaxial pattern may be less than a second semiconductorelement concentration in the bulk epitaxial pattern. The base epitaxialpattern may have a first thickness on the first surface and a secondthickness on the second surface, and a ratio of the second thickness tothe first thickness of the base epitaxial pattern may be in a rangeabout ¾ to about 1.

According to various embodiments, the substrate may include firstdopants of a first conductivity type and the bulk epitaxial pattern mayinclude second dopants of a second conductivity type different from thefirst conductivity type. The base epitaxial pattern may be free of thesecond dopants or a second dopant concentration in the base epitaxialpattern may be less than a second dopant concentration in the bulkepitaxial pattern.

In various embodiments, forming the recess region may include performingan anisotropic dry etching process to form a concave region in thesubstrate at a side of the gate pattern and performing an anisotropicwet etching process in the concave region to form the recess region. Theanisotropic wet etching process may use {111} crystal planes of thesubstrate as etch stop surfaces.

In various embodiments, the method may further include performing anadditional selective epitaxial growth (SEG) process to form a bufferepitaxial pattern including the second semiconductor element on the baseepitaxial pattern before performing the second SEG process. A processpressure of the additional SEG process may be lower than the processpressure of the first SEG process.

According to various embodiments, a second semiconductor elementconcentration in the buffer epitaxial pattern may be less than a secondsemiconductor element concentration in the bulk epitaxial pattern andmay be greater than a second semiconductor element concentration in thebase epitaxial pattern.

A method of manufacturing an integrated circuit device may includeforming a recess in a substrate including a first element. An innersurface of the recess may include a first surface of a (100) crystalplane and a second surface of one of {111} crystal planes. The methodmay further include forming a first epitaxial layer on the inner surfaceof the recess. The method may also include forming a second epitaxiallayer in the recess on the first epitaxial layer. The first epitaxiallayer may extend between the inner surface of the recess and the secondepitaxial layer, and the second epitaxial layer may include a secondelement having a lattice size different from a lattice size of the firstelement.

In various embodiments, forming the first epitaxial layer may includeperforming an epitaxial growth process at a process pressure in a rangeof about 50 Torr to about 300 Torr.

According to various embodiments, the first epitaxial layer may includea portion of a first thickness on the first surface and a portion of asecond thickness on the second surface, and a ratio of the secondthickness to the first thickness may be in a range of about ¾ to about1.

According to various embodiments, the method of claim may furtherinclude forming a gate structure on the substrate. A portion of therecess, whose inner surface may include the second surface, may betapered toward a region under the gate structure.

In various embodiments, the first epitaxial layer may include the secondelement, and a second element concentration of the first epitaxial layermay be less than a second element concentration of the second epitaxiallayer.

In various embodiments, the second epitaxial layer may include a firstdopant of a first conductivity type, and a first dopant concentration ofthe second epitaxial layer may be greater than a first dopantconcentration of the first epitaxial layer.

According to various embodiments, the first epitaxial layer may be freeof the first dopant.

In various embodiments, the substrate may include a second dopant of asecond conductivity type opposite to the first conductivity type.

According to various embodiments, the method of claim may furtherinclude forming a third epitaxial layer on the first epitaxial layerbefore forming the second epitaxial layer.

According to various embodiments, forming the first epitaxial layer mayinclude performing a first epitaxial growth process at a first processpressure and forming the third epitaxial layer may include performing asecond epitaxial growth process at a second process pressure less thanthe first process pressure.

In various embodiments, the first epitaxial layer may include a portionof a first thickness on the first surface and a portion of a secondthickness on the second surface, and the third epitaxial layer mayinclude a portion of a third thickness on the first surface and aportion of a fourth thickness on the second surface. A ratio of thesecond thickness to the first thickness may be greater than a ratio ofthe fourth thickness to the third thickness.

According to various embodiments, the first and third epitaxial layersmay include the second element, and a second element concentration ofthe third epitaxial layer may be greater than a second elementconcentration of the first epitaxial layer and may be less than a secondelement concentration of the second epitaxial layer.

According to various embodiments, the second and third epitaxial layersmay include a dopant of a first conductivity type and a dopantconcentration of the second epitaxial layer may be greater than a dopantconcentration of the third epitaxial layer.

In various embodiments, the first epitaxial layer may be free of thedopant or a dopant concentration of the first epitaxial layer may beless than the dopant concentration of the third epitaxial layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts;

FIG. 1B is an enlarged view of a portion ‘A’ of FIG. 1A;

FIG. 2A is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts;

FIG. 2B is an enlarged view of a portion ‘B’ of FIG. 2A;

FIG. 3 is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts;

FIGS. 4 to 9 are cross-sectional views illustrating a method ofmanufacturing a semiconductor device according to some embodiments ofthe inventive concepts;

FIGS. 10 and 11 are cross-sectional views illustrating a modifiedexample of a method of manufacturing a semiconductor device according tosome embodiments of the inventive concepts;

FIGS. 12 to 17 are cross-sectional views illustrating another modifiedexample of a method of manufacturing a semiconductor device according tosome embodiments of the inventive concepts;

FIG. 18 is a block diagram illustrating an example of electronic systemsincluding semiconductor devices according to some embodiments of theinventive concepts; and

FIG. 19 is a block diagram illustrating an example of memory cardsincluding semiconductor devices according to some embodiments of theinventive concepts.

DETAILED DESCRIPTION

Example embodiments are described below with reference to theaccompanying drawings. Many different forms and embodiments are possiblewithout deviating from the spirit and teachings of this disclosure andso the disclosure should not be construed as limited to the exampleembodiments set forth herein. Rather, these example embodiments areprovided so that this disclosure will be thorough and complete, and willconvey the scope of the disclosure to those skilled in the art. In thedrawings, the sizes and relative sizes of layers and regions may beexaggerated for clarity. Like reference numbers refer to like elementsthroughout.

Example embodiments of the inventive concepts are described herein withreference to cross-sectional illustrations that are schematicillustrations of idealized embodiments and intermediate structures ofexample embodiments. As such, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected. Thus, example embodiments of theinventive concepts should not be construed as limited to the particularshapes illustrated herein but include deviations in shapes that result,for example, from manufacturing. For example, an implanted regionillustrated as a rectangle may have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the embodiments.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including,” when used in thisspecification, specify the presence of the stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

It will be understood that when an element is referred to as being“coupled,” “connected,” or “responsive” to, or “on,” another element, itcan be directly coupled, connected, or responsive to, or on, the otherelement, or intervening elements may also be present. In contrast, whenan element is referred to as being “directly coupled,” “directlyconnected,” or “directly responsive” to, or “directly on,” anotherelement, there are no intervening elements present. As used herein theterm “and/or” includes any and all combinations of one or more of theassociated listed items.

It will be understood that although the terms first, second, etc. may beused herein to describe various elements, these elements should not belimited by these terms. These terms are only used to distinguish oneelement from another. Thus, a first element could be termed a secondelement without departing from the teachings of the present embodiments.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein may be interpreted accordingly.

FIG. 1A is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts, and FIG. 1B isan enlarged view of a portion ‘A’ of FIG. 1A.

Referring to FIG. 1A, gate patterns 110 may be disposed on a substrate100. The substrate 100 may include a first semiconductor element. Forexample, the substrate 100 may be formed of silicon. In other words, thefirst semiconductor element may be silicon. The substrate 100 may have asingle-crystalline state. The substrate 100 may be doped with dopants ofa first conductivity type. Each of the gate patterns 110 may include agate insulating layer 102, a gate electrode 104, and a cappinginsulation pattern 106 which are sequentially stacked on the substrate100. In some embodiments, the capping insulation pattern 106, the gateelectrode 104, and the gate insulating layer 102 in the gate pattern 110may have sidewalls aligned with each other, respectively. For example,the gate insulating layer 102 may include at least one of silicon oxide,silicon nitride, silicon oxynitride, and high-k dielectric (e.g., aninsulating metal oxide such as aluminum oxide and/or hafnium oxide). Thegate electrode 104 may include at least one of a doped semiconductormaterial (e.g., doped silicon), a metal (e.g., tungsten, titanium,and/or tantalum), a conductive metal nitride (e.g., titanium nitride,tantalum nitride, and/or tungsten nitride), and a metal-semiconductorcompound (e.g., a metal silicide). The capping insulation pattern 106may include silicon nitride and/or silicon oxynitride.

Gate spacers 115 may be disposed on both sidewalls of each of the gatepatterns 110, respectively. For example, the gate spacer 115 may includesilicon oxide, silicon nitride, and/or silicon oxynitride.

Recess regions 120 may be disposed in the substrate 100 adjacent to bothsides of the gate pattern 110. An inner surface of each of the recessregions 120 may include a first surface 122 and a second surface 124.The first surface 122 is a (100) crystal plane of the substrate 100. Thesecond surface 124 is one of {111} crystal planes of the substrate 100.The inner surface of each of the recess regions 120 may include aplurality of {111} crystal planes. As illustrated in FIG. 1A, each ofthe recess regions 120 may include an undercut region tapered toward achannel region 114 under the gate pattern 110. Ends of two {111} crystalplanes may be in contact with each other to define the tapered undercutregion.

A base epitaxial pattern 130 may be disposed on the inner surface ofeach of the recess regions 120. The base epitaxial pattern 130 mayextend along the inner surface of the recess region 120. The baseepitaxial pattern 130 may be in contact with the inner surface of therecess region 120. A bulk epitaxial pattern 135 may be disposed on thebase epitaxial pattern 130 so as to fill the recess region 120. In someembodiments, the bulk epitaxial pattern 135 may be in contact with thebase epitaxial pattern 130.

The base epitaxial pattern 130 and the bulk epitaxial pattern 135include a second semiconductor element different from the firstsemiconductor element of the substrate 100. An atomic diameter of thesecond semiconductor element is different from an atomic diameter of thefirst semiconductor element. Thus, lattice sizes of the base and bulkepitaxial patterns 130 and 135 may be different from a lattice size ofthe substrate 100. As a result, the base and bulk epitaxial patterns 130and 135 in the tapered undercut region may apply a specific force (e.g.,a compressive force or a tensile force) to the channel region 114, suchthat mobility of charges may increase in the channel region 114.

Referring to FIGS. 1A and 1B, the base epitaxial pattern 130 has a firstthickness T1 on the first surface 122 of the recess region 120 and asecond thickness T2 on the second surface 124 of the recess region 120.A ratio of the second thickness T2 to the first thickness T1 may have arange of about 0.75:1 to about 1:1. In other words, the second thicknessT2 may have a range of about 75% to about 100% of the first thicknessT1. Thus, the base epitaxial pattern 130 may be substantiallyconformally disposed along the inner surface of the recess region 120.As a result, a volume of the bulk epitaxial pattern 135 filling therecess region 120 may be maximized. A concentration of the secondsemiconductor element of the bulk epitaxial pattern 135 is greater thana concentration of the second semiconductor element of the baseepitaxial pattern 130. Thus, the specific force applied by the bulkepitaxial pattern 135 may increase, and the base epitaxial pattern 130may relax stress caused by difference between the lattice sizes of thebulk epitaxial pattern 135 and the inner surface of the recess region120.

As described above, the first surface 122 of the recess region 120 isthe (100) crystal plane and the second surface 124 is one of the {111}crystal planes. Generally, a growth rate of an epitaxial layer on the(100) crystal plane may be greater than a growth rate of an epitaxiallayer on the {111} crystal planes, such that an epitaxial layer on the(100) crystal plane may be thicker than an epitaxial layer on the {111}crystal planes. Thus, if the epitaxial layer on the {111} crystal planesis thick enough to relax the stress, the epitaxial layer on the (100)crystal plane may be thicker. Therefore, a volume of a bulk epitaxiallayer may be reduced such that a specific force applied to a channelregion 114 may be reduced.

However, according to some embodiments of the inventive concepts, it ispossible to reduce or minimize difference between a growth rate of thebase epitaxial pattern 130 on the second surface 124 and a growth rateof the base epitaxial pattern 130 on the first surface 122, so that theratio of the second thickness T2 to the first thickness T1 of the baseepitaxial pattern 130 has the range of about 0.75:1 to about 1:1. Thus,the base epitaxial pattern 130 may be substantially conformally formedon the inner surface of the recess region 120, and thus the volume ofthe bulk epitaxial pattern 135 may be increased or maximized. As aresult, the specific force applied to the channel region 114 may beincreased while the base epitaxial pattern 130 may relax the stressbetween the bulk epitaxial pattern 135 and the inner surface of therecess region 120. The base epitaxial pattern 130 may be formed by aselective epitaxial growth (SEG) process under a high process pressurein a range of about 50 Torr to about 300 Torr to reduce the differencebetween the growth rate of the base epitaxial pattern 130 on the secondsurface 124 and the growth rate of the base epitaxial pattern 130 on thefirst surface 122.

If a transistor including the channel region 114 and the gate pattern110 is a PMOS transistor, the base and bulk epitaxial patterns 130 and135 may apply the compressive force to the channel region 114. Thus,mobility of holes may be increased in a channel generated in the channelregion 114. The atomic diameter of the second semiconductor element ofthe base and bulk epitaxial patterns 130 and 135 may be greater than theatomic diameter of the first semiconductor element of the substrate 100.For example, if the substrate is a silicon substrate, the base and bulkepitaxial patterns 130 and 135 may include silicon-germanium (SiGe). Inthis case, the second semiconductor element may be germanium (Ge).

Alternatively, if the transistor including the channel region 114 andthe gate pattern 110 is a NMOS transistor, the base and bulk epitaxialpatterns 130 and 135 may apply the tensile force to the channel region114. Thus, mobility of electrons may be increased in the channelgenerated in the channel region 114. The atomic diameter of the secondsemiconductor element of the base and bulk epitaxial patterns 130 and135 may be smaller than the atomic diameter of the first semiconductorelement of the substrate 100. For example, if the substrate 100 is asilicon substrate, the base and bulk epitaxial patterns 130 and 135 mayinclude silicon carbide (SiC). In this case, the second semiconductorelement may be carbon (C).

The substrate 100 is doped with dopants of the first conductivity type,and the bulk epitaxial patterns 135 are doped with dopants of a secondconductivity type different from the first conductivity type. The bulkepitaxial patterns 135 may correspond to source/drain regions of thetransistor. One of the first and second conductivity types is an N-typeand another of the first and second conductivity types is a P-type. Thebulk epitaxial pattern 135 may be heavily doped.

The base epitaxial pattern 130 may have a dopant concentration less thana concentration of the dopants of the second conductivity type in thebulk epitaxial pattern 135. For example, the base epitaxial pattern 130may be undoped. Alternatively, the base epitaxial pattern 130 may bedoped with dopants of the second conductivity type and have aconcentration of the dopants of the second conductivity type less thanthe concentration of the dopants of the second conductivity type in thebulk epitaxial pattern 135. Since the base epitaxial pattern 130 isundoped or has the low concentration of the dopants of the secondconductivity type, a leakage current of the transistor may be reduced.

In an embodiment, a low concentration doped region 112 may be disposedin the substrate 100 between the channel region 114 and the baseepitaxial pattern 130. The low concentration doped region 112 is dopedwith dopants of the second conductivity. A concentration of the dopantsin the low concentration doped region 112 may be less than theconcentration of the dopants in the bulk epitaxial pattern 135. Thus,the low concentration doped region and the bulk epitaxial pattern 135may be realized as a Lightly Doped Drains (LDD) source/drain region oran extension source/drain.

A capping epitaxial pattern 140 may be disposed on each of the bulkepitaxial patterns 135. A top surface of the capping epitaxial pattern140 may be higher than a top surface of the substrate 100. For example,the capping epitaxial pattern 140 may be formed of silicon. An ohmicpattern 145 may be disposed on the capping epitaxial pattern 140. Forexample, the ohmic pattern 145 may be formed of a metal-semiconductorcompound (e.g., a metal silicide). For example, the ohmic pattern 145may be formed by reaction between a metal and the capping epitaxialpattern 140. The ohmic pattern 145 may be formed of, for example, cobaltsilicide, nickel silicide, and/or titanium silicide.

Next, modified examples of the semiconductor device according to someembodiments of the inventive concepts will be described with referenceto the drawings. For the purpose of ease and convenience, thedescriptions of the same components as in the above embodiment will beomitted or mentioned briefly.

FIG. 2A is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts, and FIG. 2B isan enlarged view of a portion ‘B’ of FIG. 2A.

Referring to FIGS. 2A and 2B, a buffer epitaxial pattern 133 may bedisposed between the base epitaxial pattern 130 and the bulk epitaxialpattern 135. The buffer epitaxial pattern 133 includes the secondsemiconductor element. For example, if the transistor is the PMOStransistor, the buffer epitaxial pattern 133 may includesilicon-germanium (SiGe). If the transistor is the NMOS transistor, thebuffer epitaxial pattern 133 may include silicon carbide (SiC).

A concentration of the second semiconductor element of the bufferepitaxial pattern 133 may be less than the concentration of the secondsemiconductor element of the bulk epitaxial pattern 135. In someembodiments, the concentration of the second semiconductor element ofthe buffer epitaxial pattern 133 may be greater than the concentrationof the second semiconductor element of the base epitaxial pattern 130.

As illustrated in FIG. 2B, the buffer epitaxial pattern 133 may have athird thickness Ta on the first surface 122 of the recess region 120 anda fourth thickness Tb on the second surface 124 of the recess region120. A ratio of the fourth thickness Tb to the third thickness Ta of thebuffer epitaxial pattern 133 may be less than the ratio of the secondthickness T2 to the first thickness T1 of the base epitaxial pattern130. In other words, the ratio of the fourth thickness Tb to the thirdthickness Ta of the buffer epitaxial pattern 133 may be less than about0.75:1.

Referring to FIGS. 2A and 2B, the buffer epitaxial pattern 133 may bedoped with dopants of the second conductivity type. A concentration ofthe dopants of the buffer epitaxial pattern 133 may be less than theconcentration of the dopants of the bulk epitaxial pattern 135. Thedopant concentration of the base epitaxial pattern 133 may be equal to 0(zero) and or may be less than the concentration of the dopants of thebuffer epitaxial pattern 133.

FIG. 3 is a cross-sectional view illustrating a semiconductor deviceaccording to some embodiments of the inventive concepts. In the presentmodified example, a gate pattern may have a structure different fromthat of the gate pattern 110 of FIGS. 1A and 2A.

Referring to FIG. 3, a gate pattern 180 may include a gate insulatingpattern 165 a and a gate electrode which are sequentially stacked on thesubstrate 100. In an embodiment, the gate electrode may include abarrier conductive pattern 170 a and a metal pattern 175 a which aresequentially stacked. Both ends of the barrier conductive pattern 170 amay extend upward to cover both sidewalls of the metal pattern 175 a,respectively. Both ends of the gate insulating pattern 165 a may extendupward to cover both sidewalls of the gate electrode, respectively. Eachof the extending portions of the barrier conductive pattern 170 a may bedisposed between the metal pattern 175 and each of the extendingportions of the gate insulating pattern 165 a.

A top surface of the gate pattern 180 may be substantially coplanar withtop surfaces of gate spacers 115 a and an interlayer insulating layer150 a which are disposed at both sides of the gate pattern 180. Theinterlayer insulating layer 150 a may cover the ohmic pattern 145. Thegate insulating pattern 165 a may include at least one of silicon oxide,silicon nitride, silicon oxynitride, and a high-k dielectric (e.g., aninsulating metal oxide such as hafnium oxide and/or aluminum oxide). Thebarrier conductive pattern 170 a may include a conductive metal nitride(e.g., titanium nitride, tantalum nitride, and/or tungsten nitride). Themetal pattern 175 a may include tungsten and/or aluminum.

The gate pattern 180 according to the present modified example may bereplaced with the gate pattern 110 of FIG. 1A.

The semiconductor devices according to some may be various kinds ofsemiconductor devices such as semiconductor memory devices, logicdevices, and system on chips (SOCs).

FIGS. 4 to 9 are cross-sectional views illustrating a method ofmanufacturing a semiconductor device according to some embodiments ofthe inventive concepts.

Referring to FIG. 4, gate patterns 110 may be formed on a substrate 100which is formed of a first semiconductor element and is doped withdopants of a first conductivity type. Each of the gate patterns 110 mayinclude a gate insulating layer 102, a gate electrode 104, and a cappinginsulating pattern 106 which are sequentially stacked on the substrate100. Dopants of a second conductivity type may be implanted into thesubstrate 100 at both sides of the gate pattern 110, thereby forming lowconcentration doped regions 112. Gate spacers 115 may be formed on bothsidewalls of each of the gate patterns 110, respectively.

Referring to FIG. 5, an anisotropic dry etching process may be performedon the substrate 100 using the capping insulating patterns 106 and thegate spacers 155 as etch masks. Thus, concave regions 117 may be formedin the substrate 100 at both sides of each of the gate patterns 110.

Referring to FIG. 6, an anisotropic wet etching process may be performedin concave regions 117 to form recess regions 120. The anisotropic wetetching process may use {111} crystal planes of crystal planes of thesubstrate 100 as etch stop surfaces. In other words, the anisotropic wetetching process may etch the {111} crystal planes very slowly comparedwith other crystal planes of the substrate 100. Sidewalls of the concaveregions 117 may be laterally etched by the anisotropic wet etchingprocess to form the recess regions 120 including tapered undercutregions under the gate pattern 110. The tapered undercut region may belaterally tapered toward a channel region 114 under the gate pattern110. Due to the anisotropic wet etching process, an inner surface of therecess region 120 includes a first surface 122 of a (100) crystal planeand a second surface 124 of one of the {111} crystal planes. The firstsurface 122 may correspond to a bottom surface of the recess region 120.

In some embodiments, if the substrate 100 is a silicon substrate, theanisotropic wet etching process may include an anisotropic etchantincluding ammonium hydroxide (NH4OH) and/or tetramethyl ammoniumhydroxide (TMAH).

Referring to FIG. 7, a first selective epitaxial growth (SEG) processmay be performed to form a base epitaxial pattern 130 on the innersurface of the recess region 120. The first SEG process may be performedunder a high process pressure in a range of about 50 Torr to about 300Torr. The first SEG process may use a process gas including asemiconductor source gas. The semiconductor source gas may include asecond semiconductor element different from the first semiconductorelement. Additionally, the semiconductor source gas may further includethe first semiconductor element. For example, if a transistor includingthe gate pattern 110 is a PMOS transistor, the semiconductor source gasmay include silicon and germanium. If the transistor is a NMOStransistor, the semiconductor source gas may include silicon and carbon.

Since the first SEG process is performed under the high processpressure, the semiconductor source gas may be sufficiently supplied tothe second surface 124 (i.e., one of the {111} crystal planes) of therecess region 120. Thus, it is possible to reduce or minimize differencebetween a growth rate of the base epitaxial pattern 130 on the secondsurface 124 and a growth rate of the base epitaxial pattern 130 on thefirst surface 122. As a result, the base epitaxial pattern 130 may besubstantially conformally formed on the inner surface of the recessregion 120. In other words, a ratio of a thickness of the base epitaxialpattern 130 on the second surface 124 to a thickness of the baseepitaxial pattern 130 on the first surface 122 may be in a range ofabout 0.75:1 to about 1:1.

The base epitaxial pattern 130 may be undoped. Alternatively, the baseepitaxial pattern 130 may be lightly doped with dopants of a secondconductivity type different from the first conductivity type. In thiscase, the base epitaxial pattern 130 may be doped in-situ. In otherwords, the process gas of the first SEG process may further include adopant source gas having dopants of the second conductivity type.

Referring to FIG. 8, a second selective epitaxial growth (SEG) processmay be performed on the base epitaxial pattern 130 to form a bulkepitaxial pattern 135. The bulk epitaxial pattern 135 may fill therecess region 120 on the base epitaxial pattern 130. A process gas ofthe second SEG process includes a semiconductor source gas. Thesemiconductor source gas of the second SEG process includes the secondsemiconductor element. Additionally, the semiconductor source gas of thesecond SEG process may further include the first semiconductor element.For example, if the transistor is the PMOS transistor, the semiconductorsource gas of the second SEG process may include silicon and germanium.If the transistor is the NMOS transistor, the semiconductor source gasof the second SEG process may include silicon and carbon.

The amount of the second semiconductor element in the semiconductorsource gas of the second SEG process is greater than the amount of thesecond semiconductor element in the semiconductor source gas of thefirst SEG process. Thus, a concentration of the second semiconductorelement in the bulk epitaxial pattern 135 may be greater than aconcentration of the second semiconductor element in the base epitaxialpattern 130.

The bulk epitaxial pattern 135 is doped with dopants of the secondconductivity type. The bulk epitaxial pattern 135 may be doped in-situ.For example, the process gas of the second SEG process may furtherinclude a dopant source gas having dopants of the second conductivitytype. The bulk epitaxial pattern 135 may be heavily doped with thedopants of the second conductivity type. A process pressure of thesecond SEG process may be in a range of about 10 Torr to about 100 Torr.The first and second SEG processes may be sequentially performed in oneprocess chamber.

Referring to FIG. 9, a third selective epitaxial growth (SEG) processmay be performed on the bulk epitaxial pattern 135 to form a cappingepitaxial pattern 140. For example, the capping epitaxial pattern 140may be formed of the first semiconductor element. For example, thecapping epitaxial pattern 140 may be formed of silicon. The cappingepitaxial pattern 140 may be doped with dopants of the secondconductivity type. Subsequently, a metal layer may be formed on thesubstrate 100, and the metal layer may react with the capping epitaxialpattern 140, thereby forming the ohmic pattern 145 of FIG. 1A.

According to the method of manufacturing the semiconductor devicedescribed above, the base epitaxial pattern 130 is formed under the highprocess pressure. Thus, it is possible to reduce the difference betweenthe growth rate of the base epitaxial pattern 130 on the second surface124 and the growth rate of the base epitaxial pattern 130 on the firstsurface 122. As a result, the base epitaxial pattern 130 may besubstantially conformally formed on the inner surface of the recessregion 120.

FIGS. 10 and 11 are cross-sectional views illustrating a method ofmanufacturing a semiconductor device according to some embodiments ofthe inventive concepts.

Referring to FIGS. 7 and 10, an additional selective epitaxial growth(SEG) process may be performed on the base epitaxial pattern 130 to forma buffer epitaxial pattern 133. A process pressure of the additional SEGprocess may be lower than the process pressure of the first SEG process.Thus, a ratio of a thickness of the buffer epitaxial pattern 133 on thesecond surface 124 to a thickness of the buffer epitaxial pattern 133 onthe first surface 122 may be lower than the ratio of the thickness ofthe base epitaxial pattern 130 on the second surface 124 to thethickness of the base epitaxial pattern 130 on the first surface 122.For example, the process pressure of the additional SEG process may bein a range of about 10 Torr to 30 Torr.

A process gas of the additional SEG process may include the secondsemiconductor element. Additionally, the process gas of the additionalSEG process may further include the first semiconductor element. Thebuffer epitaxial pattern 133 may be doped with dopants of the secondconductivity type. For example, the buffer epitaxial pattern 133 may bedoped in-situ. In this case, the process gas of the additional SEGprocess may further include a dopant source gas including dopants of thesecond conductivity type.

Referring to FIG. 11, the SEG process may be performed on the bufferepitaxial pattern 133 to form a bulk epitaxial pattern 135 on the bufferepitaxial pattern 133.

The amount of the second semiconductor element in the semiconductorsource gas of the additional SEG process may be greater than the amountof the second semiconductor element in the semiconductor source gas ofthe first SEG process and less than the amount of the secondsemiconductor element in the semiconductor source of the second SEGprocess. Thus, a concentration of the second semiconductor element inthe buffer epitaxial pattern 133 may be greater than the concentrationof the second semiconductor element in the base epitaxial pattern 130and less than the concentration of the second semiconductor element inthe bulk epitaxial pattern 135.

The third SEG process described with reference to FIG. 9 may beperformed to form the capping epitaxial pattern 140 and the ohmicpattern 145 of FIG. 2A may be formed on the capping epitaxial pattern140.

FIGS. 12 to 17 are cross-sectional views illustrating a method ofmanufacturing a semiconductor device according to exemplary embodimentsof the inventive concepts.

Referring to FIG. 12, dummy gate patterns 210 may be formed on asubstrate 100. Low concentration doped regions 112 may be formed in thesubstrate at both sides of each of the dummy gate patterns 210. Gatespacers 115 may be formed on both sidewalls of each of the dummy gatepatterns 210, respectively. The dummy gate pattern 210 may include amaterial having an etch selectivity with respect to the gate spacer 115and an interlayer insulating layer 150 formed through a subsequentprocess. In some embodiments, the dummy gate pattern 210 may include alower pattern 205 and an upper pattern 207 which are sequentiallystacked on the substrate 100. For example, if the gate spacer 115 isformed of silicon nitride and the interlayer insulating layer 150 isformed of silicon oxide, the lower pattern 205 may be formed of asemiconductor material (e.g., silicon) and the upper pattern 207 may beformed of silicon oxide. In some embodiments, a buffer oxide layer (notillustrated) may be formed between the dummy gate pattern 210 and thesubstrate 100.

Referring to FIG. 13, the recess regions 120 may be formed in thesubstrate 100 at both sides of each of the dummy gate patterns 210,respectively. The recess regions 120 may be formed by the processesdescribed with reference to FIGS. 5 and 6.

Referring to FIG. 14, the base, buffer, and bulk epitaxial patterns 130,133, and 135 may be sequentially formed in each of the recess regions120. In some embodiments, the formation of buffer epitaxial pattern 133may be omitted. The capping epitaxial pattern 140 may be formed on thebulk epitaxial pattern 135, and the ohmic pattern 145 may be formed onthe capping epitaxial pattern 140. The interlayer insulating layer 150may be formed on an entire surface of the substrate 100.

Referring to FIG. 15, the interlayer insulating layer 150 and the upperpattern 207 of the dummy gate pattern 210 may be planarized until thelower pattern 205 is exposed. Upper portions of the gate spacers 115 mayalso be planarized by the planarizing process. As mentioned above, thelower pattern 205 of the dummy gate pattern 210 may have the etchselectivity with respect to the planarized interlayer insulating layer150 a and the planarized gate spacers 115 a.

Referring to FIG. 16, the exposed lower patterns 205 may be removed toform openings 160. If the buffer oxide layer is formed, the buffer oxidelayer may be removed to expose the substrate 100 under the openings 160after the removal of the lower patterns 205.

Referring to FIG. 17, a gate insulating layer 165 and a gate conductivelayer may be sequentially formed on the substrate 100 having theopenings 160. In some embodiments, the gate conductive layer may includea barrier conductive layer 170 and a metal layer 175 which aresequentially stacked. The gate insulating layer 165 may be formed by achemical vapor deposition (CVD) process or an atomic layer deposition(ALD) process. Thus, the gate insulating layer 165 may be substantiallyconformally formed on the substrate 100. Alternatively, the gateinsulating layer 165 may be formed by an oxidation process and/or anitridation process. In this case, the gate insulating layer 165 may beselectively formed on the substrate 100 exposed by each of the openings160.

The metal layer 175, the barrier conductive layer 170, and the gateinsulating layer 165 may be planarized until the planarized interlayerinsulating layer 150 a is exposed. Thus, the gate patterns 180 of FIG. 3may be formed. As described above, the gate pattern 180 may include thegate insulating pattern 165 a, the barrier conductive pattern 170 a, andthe metal pattern 175 a.

The semiconductor devices described above may be encapsulated usingvarious packaging techniques. For example, the semiconductor devicesaccording to the aforementioned embodiments may be encapsulated usingany one of a package on package (POP) technique, a ball grid arrays(BGAs) technique, a chip scale packages (CSPs) technique, a plasticleaded chip carrier (PLCC) technique, a plastic dual in-line package(PDIP) technique, a die in waffle pack technique, a die in wafer formtechnique, a chip on board (COB) technique, a ceramic dual in-linepackage (CERDIP) technique, a plastic metric quad flat package (PMQFP)technique, a plastic quad flat package (PQFP) technique, a small outlinepackage (SOIC) technique, a shrink small outline package (SSOP)technique, a thin small outline package (TSOP) technique, a thin quadflat package (TQFP) technique, a system in package (SIP) technique, amulti chip package (MCP) technique, a wafer-level fabricated package(WFP) technique and a wafer-level processed stack package (WSP)technique.

The package in which the semiconductor device according to someembodiments is mounted may further include at least one semiconductordevice (e.g., a controller and/or a logic device) that controls thesemiconductor memory device.

FIG. 18 is a block diagram illustrating an example of electronic systemsincluding semiconductor devices according to some embodiments of theinventive concepts.

Referring to FIG. 18, an electronic system 1100 according to anembodiment may include a controller 1110, an input/output (I/O) unit1120, a memory device 1130, an interface unit 1140 and a data bus 1150.At least two of the controller 1110, the I/O unit 1120, the memorydevice 1130 and the interface unit 1140 may communicate with each otherthrough the data bus 1150. The data bus 1150 may correspond to a paththrough which electrical signals are transmitted.

The controller 1110 may include at least one of a microprocessor, adigital signal processor, a microcontroller or another logic device. Theother logic device may have a similar function to any one of themicroprocessor, the digital signal processor and the microcontroller.The I/O unit 1120 may include a keypad, a keyboard and/or a displayunit. The memory device 1130 may store data and/or commands. The memorydevice 1130 and/or the controller 1110 may include at least one of thesemiconductor devices according to some embodiments described above.

The interface unit 1140 may transmit electrical data to a communicationnetwork or may receive electrical data from a communication network. Theinterface unit 1140 may operate by wireless or cable. For example, theinterface unit 1140 may include an antenna for wireless communication ora transceiver for cable communication. The electronic system 1100 mayfurther include a fast DRAM device and/or a fast SRAM device which actsas a cache memory for improving an operation of the controller 1110.

The electronic system 1100 may be applied to a personal digitalassistant (PDA), a portable computer, a web tablet, a wireless phone, amobile phone, a digital music player, a memory card or other electronicproducts. The other electronic products may receive or transmitinformation data by wireless.

FIG. 19 is a block diagram illustrating an example of memory cardsincluding semiconductor devices according to some embodiments of theinventive concepts.

Referring to FIG. 19, a memory card 1200 according to an embodiment ofthe inventive concepts may include a memory device 1210. Thesemiconductor devices according to the aforementioned embodimentscomprise semiconductor memory devices, the memory device 1210 mayinclude at least one of the semiconductor devices according to someembodiments mentioned above. The memory card 1200 may include a memorycontroller 1220 that controls data communication between a host and thememory device 1210.

The memory controller 1220 may include a central processing unit (CPU)1222 that controls overall operations of the memory card 1200. Inaddition, the memory controller 1220 may include an SRAM device 1221 asan operation memory of the CPU 1222. Moreover, the memory controller1220 may further include a host interface unit 1223 and a memoryinterface unit 1225. The host interface unit 1223 may be configured toinclude a data communication protocol between the memory card 1200 andthe host. The memory interface unit 1225 may connect the memorycontroller 1220 to the memory device 1210. The memory controller 1220may further include an error check and correction (ECC) block 1224. TheECC block 1224 may detect and correct errors of data which are read outfrom the memory device 1210. The memory card 1200 may further include aread only memory (ROM) device that stores code data to interface withthe host. The memory card 1200 may be used as a portable data storagecard. Alternatively, the memory card 1200 may be solid state disks (SSD)which are used as hard disks of computer systems.

According to various embodiments of the inventive concepts as describedabove, the ratio of the second thickness to the first thickness of thebase epitaxial pattern may have the range of about 0.75:1 to about 1:1.Thus, the base epitaxial pattern may be substantially conformally formedon the inner surface of the recess region. As a result, the volume ofthe bulk epitaxial pattern may increase in the recess region, such thatthe specific force (e.g., the compressive force or the tensile force)may be sufficiently applied to the channel region under the gatepattern.

The first SEG process for the formation of the base epitaxial patternmay be performed under the high process pressure of about 50 Torr toabout 300 Torr. Thus, the semiconductor source gas may be sufficientlysupplied to the {111} crystal planes of the inner surface of the recessregion. As a result, it is possible to improve uniformity of thethickness of the base epitaxial pattern.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments, which fall withinthe true spirit and scope of the inventive concepts. Thus, to themaximum extent allowed by law, the scope is to be determined by thebroadest permissible interpretation of the following claims and theirequivalents, and shall not be restricted or limited by the foregoingdetailed description.

1.-9. (canceled)
 10. A method of manufacturing a semiconductor devicecomprising: forming a gate pattern on a substrate comprising a firstsemiconductor element; forming a recess region in the substrate at aside of the gate pattern, an inner surface of the recess regionincluding a first surface of a (100) crystal plane and a second surfaceof one of {111} crystal planes; performing a first selective epitaxialgrowth (SEG) process to form a base epitaxial pattern on the innersurface of the recess region at a process pressure in a range of about50 Torr to about 300 Torr, the base epitaxial pattern comprising asecond semiconductor element different from the first semiconductorelement; and performing a second selective epitaxial growth (SEG)process to form a bulk epitaxial pattern comprising the secondsemiconductor element on the base epitaxial pattern.
 11. The method ofclaim 10, wherein a second semiconductor element concentration in thebase epitaxial pattern is less than a second semiconductor elementconcentration in the bulk epitaxial pattern, and wherein the baseepitaxial pattern has a first thickness on the first surface and asecond thickness on the second surface, and a ratio of the secondthickness to the first thickness of the base epitaxial pattern is in arange about ¾ to about
 1. 12. The method of claim 10, wherein thesubstrate comprises first dopants of a first conductivity type and thebulk epitaxial pattern comprises second dopants of a second conductivitytype different from the first conductivity type, and wherein the baseepitaxial pattern is free of the second dopants or a second dopantconcentration in the base epitaxial pattern is less than a second dopantconcentration in the bulk epitaxial pattern.
 13. The method of claim 10,wherein forming the recess region comprises: performing an anisotropicdry etching process to form a concave region in the substrate at a sideof the gate pattern; and performing an anisotropic wet etching processin the concave region to form the recess region, wherein the anisotropicwet etching process uses {111} crystal planes of the substrate as etchstop surfaces.
 14. The method of claim 10, further comprising:performing an additional selective epitaxial growth (SEG) process toform a buffer epitaxial pattern comprising the second semiconductorelement on the base epitaxial pattern before performing the second SEGprocess, wherein a process pressure of the additional SEG process islower than the process pressure of the first SEG process.
 15. The methodof claim 14, wherein a second semiconductor element concentration in thebuffer epitaxial pattern is less than a second semiconductor elementconcentration in the bulk epitaxial pattern and is greater than a secondsemiconductor element concentration in the base epitaxial pattern.
 16. Amethod of manufacturing an integrated circuit device comprising: forminga recess in a substrate comprising a first element, wherein an innersurface of the recess comprises a first surface of a (100) crystal planeand a second surface of one of {111} crystal planes; forming a firstepitaxial layer on the inner surface of the recess; and forming a secondepitaxial layer in the recess on the first epitaxial layer, wherein thefirst epitaxial layer extends between the inner surface of the recessand the second epitaxial layer, and wherein the second epitaxial layercomprises a second element having a lattice size different from alattice size of the first element.
 17. The method of claim 16, whereinforming the first epitaxial layer comprises performing an epitaxialgrowth process at a process pressure in a range of about 50 Torr toabout 300 Torr.
 18. The method of claim 16, wherein the first epitaxiallayer comprises a portion of a first thickness on the first surface anda portion of a second thickness on the second surface, and wherein aratio of the second thickness to the first thickness is in a range ofabout ¾ to about
 1. 19. The method of claim 16, further comprisingforming a gate structure on the substrate, wherein a portion of therecess, whose inner surface comprises the second surface, is taperedtoward a region under the gate structure.
 20. The method of claim 16,wherein the first epitaxial layer comprises the second element, andwherein a second element concentration of the first epitaxial layer isless than a second element concentration of the second epitaxial layer.21. The method of claim 16, wherein the second epitaxial layer comprisesa first dopant of a first conductivity type, and wherein a first dopantconcentration of the second epitaxial layer is greater than a firstdopant concentration of the first epitaxial layer.
 22. The method ofclaim 21, wherein the first epitaxial layer is free of the first dopant.23. The method of claim 21, wherein the substrate comprise a seconddopant of a second conductivity type opposite to the first conductivitytype.
 24. The method of claim 16, further comprising forming a thirdepitaxial layer on the first epitaxial layer before forming the secondepitaxial layer.
 25. The method of claim 24, wherein forming the firstepitaxial layer comprises performing a first epitaxial growth process ata first process pressure and forming the third epitaxial layer comprisesperforming a second epitaxial growth process at a second processpressure less than the first process pressure.
 26. The method of claim24, wherein the first epitaxial layer comprises a portion of a firstthickness on the first surface and a portion of a second thickness onthe second surface, and the third epitaxial layer comprises a portion ofa third thickness on the first surface and a portion of a fourththickness on the second surface, and wherein a ratio of the secondthickness to the first thickness is greater than a ratio of the fourththickness to the third thickness.
 27. The method of claim 24, whereinthe first and third epitaxial layers comprise the second element, andwherein a second element concentration of the third epitaxial layer isgreater than a second element concentration of the first epitaxial layerand is less than a second element concentration of the second epitaxiallayer.
 28. The method of claim 24, wherein the second and thirdepitaxial layers comprise a dopant of a first conductivity type, andwherein a dopant concentration of the second epitaxial layer is greaterthan a dopant concentration of the third epitaxial layer.
 29. The methodof claim 28, wherein the first epitaxial layer is free of the dopant ora dopant concentration of the first epitaxial layer is less than thedopant concentration of the third epitaxial layer.